Highway Repair With Rapid Strength Concrete?

Results of a Southern California freeway surveys that set out to determine the short- and long-term performance of rapid strength concrete for the potential repair and restoration of highway infrastructure work.

Deteriorating highway and other infrastructure conditions across the U.S., combined with widespread funding shortfalls, make it clear that there is a need for cost-effective, long-term solutions for roadway preservation. Surveys of existing projects in California have addressed this need, with results documenting pavement restoration successes and offering guidance on the practice of pavement management.

The use of rapid strength concrete (RSC) for roadway construction is extremely beneficial for today’s congested highways and the traveling public. Because RSC reaches opening strength in a matter of hours, slab replacement can be accomplished overnight. The California Department of Transportation (Caltrans) has used RSC for pavement repair since the 1990s and is typically able to complete slab replacement— including removal of cracked slabs, repair of the base course, placing of dowel bars and interlayer, and placing and curing of RSC—within a six-hour window.

Two surveys were conducted, the first by Caltrans in 2008, the second in 2018-20 by myself. These surveys sought to confirm that in addition to offering the immediate benefit of short lane closures, RSC was providing durable, long-lasting repairs. Condition surveys conducted for “Evaluation of Rapid Strength Concrete Slab Repairs” (Bhattacharya, Zola, and Rawool, 2008) examined 15 freeway projects across six regions, looking at more than 5,000 “individual” RSC slabs and recording all distress. Since the slabs were only three years old at the time of the survey, the study’s goal was to document the short-term performance of RSC. Results showed that only 1.4% of the RSC slabs surveyed showed any premature distresses.

By 2018, the time was right to assess longer-term performance. The evaluation focused on two California strategies for restoration of jointed plain concrete pavement (JPCP): • “Individual” slab replacement with rapid strength concrete (ISR-RSC); • “Total lane replacement” RSC (JPCP-

RSC), (“Long-Term Performance of

Rapid Strength Concrete (RSC) Slabs on California Highways,” Darter, 2021).

For the six ISR-RSC “individual” replacement projects being assessed, cracked, or badly damaged JPCP slabs had been removed and replaced with RSC slabs along a traffic lane. RSC slab thickness (typically 8-9 in.) and joint spacing (typically 12-19 ft.) remained

A total of 3,562 RSC slabs were surveyed. Image by Mitzi McIntyre, CTS | Rapid Set.

the same and existing base courses had been largely left in place. In some cases, dowels had been included in the transverse joints. Neither shrinkage nor ASR requirements were specified at the time of slab replacement. Concrete durability issues, typically seen as surface cracks and scaling, were almost non-existent.

For the six JPCP-RSC “lane replacement” projects being assessed, all slabs and base courses along one or more traffic lanes had been removed and replaced with RSC. Slab thickness was typically increased during reconstruction to be 10-14 in.; perpendicular joint spacing was uniform at 14 ft.; and dowels were placed at all transverse joints. Both RSC shrinkage and ASR requirements were included in these specifications.

A survey team, working with Caltrans, randomly selected six of the 15 projects previously surveyed in 2008

(all ISR-RSC) to be part of a similar 2018 study. Located in southern Calif., the now 13-year-old RSC slabs had experienced on average truck traffic levels of 6 million total trucks in the inner lane and 20 million total trucks in the outer lane. A total of 1,493 ISRRSC slabs were surveyed.

An additional six projects were randomly selected to represent JPCP-RSC “lane replacement” pavements. Because only panels located in outer lanes with heavy truck traffic (16 million trucks total) had been replaced, only JPCPRSC slabs in outer lanes were included in the survey. These 2,069 RSC slabs had been in service from 9-17 years.

A total of 3,562 RSC slabs, located across 18 inner and outer traffic lanes on 12 major freeways that were two- to five lanes in each direction were surveyed. Distresses examined included transverse cracking, longitudinal cracking, corner cracking, joint spalling, shrinkage cracking, surface defects, joint faulting observations, and pumping/erosion.

MATERIALS USED & RESULTS

Rapid Set cement, manufactured by CTS Cement, was used for the majority of the original slab replacements. Rapid Set cement is a non-proprietary belitic calcium sulfoaluminate cement that reaches opening strength (400 psi flex) in 1.5 hours. It is very low shrinkage and is resistant to sulfate attack and alkali-silica reaction. Rapid Set cement can be used in a volumetric mixer or ready mixer. For a few projects, the 4x4 Concrete System was used. This proprietary system, originally patented by BASF and now offered by multiple admixture suppliers, offers opening strength (400 psi flex) in four hours but can experience higher shrinkage than Rapid Set cement if a high dosage of shrinkage-reducing admixture (SRA) is not used. It cannot be used in volumetric mixers and is appropriate for ready-mix delivery only.

ISR-RSC survey results for the “individual” slab replacements of six projects were consistent with findings from 2008: very low distress development, with one exception. Of the slabs in the outer lane, 20% had developed transverse fatigue cracks—the only significant cracking experienced across all 12 projects. Transverse cracks are the commonly seen top-down fatigue cracks caused by heavy trucks moving over the upward curled slabs and applying axle loads that cause high tensile stress in the top of the saucer-shaped slab caused by moisture and temperature gradients. Transverse fatigue cracks were predominantly located in the outer lanes, where truck traffic was heaviest. Inner lanes were found to crack at a rate of only 3%, though they carried 6 million trucks, whereas the outer lanes developed 21% carrying 16 million trucks. The survey showed only minor occurrences.

It is estimated that outer lane ISRRSC individual, thin slabs have service lives of 10-13 years and inner lanes with less truck traffic in the order of 15-20 years with the same thin slabs. The individual slab replacement appears to be a viable repair solution for lower truck traffic inner lanes, or as a short-term strategy for outer lanes. Dowel bar use at all transverse joints is recommended to prevent pumping and faulting. Replacement of the entire lane, using a closer joint spacing of 14 ft. and thicker slabs, as well as installing dowel bars at all transverse joints and providing a new base, is considered to be the most long-lived reconstruction solution. Using these construction guidelines, appropriately designed RSC pavements should achieve at least 30 years of service (based on the limited distress seen in lane replacement RSC slabs at the approximately 17-year mark). Inner lanes that experience less truck traffic are expected to last at least 30 years. Diamond grinding should be performed to achieve pavement smoothness after RSC slab replacement as needed.

“With the increasing demand that highway work be completed overnight in order to minimize impact on the traveling public and decrease emissions from traffic congestion, it’s great to receive validation that pavement management with RSC is a long term solution for our nationwide highway infrastructure,” says Ken Vallens, President and CEO of CTS Cement Manufacturing Corporation.

To date, Caltrans has not specified durability criteria for shrinkage and ASR for slab replacement work, because it has typically been considered a shortterm (approximately five-year) solution. However, with survey results showing that slab replacement using RSC can be a long-term pavement reconstruction option, it is the belief of the survey team that durability criteria should now be included.

CTS Rapid Set cement allowed slabs to be placed during the night to limit lane closures. Image by Mitzi McIntyre, CTS | Rapid Set

Mike Darter is a professor emeritus of civil and environmental engineering at the University of Illinois.

Adapted from a IGGA Technical Session: ForConstructionPros.

com/21451755.

Unveiling the BEST Contractors to Work for in Construction

There may be no better time than now to be considered a top employer in the construction industry.

Currently, the construction companies that are offering a positive workplace environment, good pay, training and opportunities for advancement are the ones that are winning the workforce shortage. The past year presented an enormous challenge for construction companies, and any employer who was able to retain and attract happy employees deserves credit. To recognize those companies, the staff of Equipment Today, Asphalt Contractor, Concrete Contractor, ForConstructionPros.com and Pavement Maintenance & Reconstruction hosted the inaugural Best Contractors to Work For recognition program, where employees were able to nominate their employers for recognition by filling out a survey. To be considered, employers needed to register, and employees filled out surveys about what it’s like to work for their boss. Entry forms and surveys were handled by the Best Companies Group, which analyzed the data and created an insights report based on the results. To be eligible, construction companies must have 15 full- or part-time employees working in the U.S. The company’s primary business activity must be either general building construction, earthmoving, paving, concrete, or specialty and trades, such as underground, utility, electrical, HVAC, demolition and others.